While installing a file, the pop up with the fake flash player downloaded a dmg to my computer. Because I was already downloading something else and expecting to find it in my downloads I accidentally opened the dmg file. At this point I knew this was not the software I wished to download so I closed the window. The installer might have been left open for several hours past that until I ejected it and deleted it however. So I opened the dmg file and left it open for a while but I didn’t install anything nor run it. Am I good? Is there anyway to be sure?
If you just opened the .dmg image and didn't manually open any file or run any program located inside it, you're fine: as far as I know, there's no "autorun" functionality on DMG images in macOS.